In next generation mobile communication and wireless transmission systems, an improved data transmission rate and system capacity needed in a multi-cell environment have been demanded. To meet such a demand, studies on a multiple-input multiple-output (MIMO) system, which transmits data using multiple antennas, have been conducted. To improve the data transmission rate in a multi-cell environment, a closed-loop MIMO system using channel information improves transmission capabilities using the channel information. To use a proper codebook according to an environment of each cell in a multi-cell environment, a plurality of codebooks is used to provide improved service to a mobile station (MS).
Generally, in a MIMO system, an MS can recognize information about a reception channel using received data, whereas a base station (BS) cannot know the channel information. To improve the performance of a system using the channel information, the BS should be aware of the channel information. When a time division duplex (TDD) scheme is used, since an uplink channel and a downlink channel between the BS and the MS are the same, the BS can know the channel information. To provide a high-quality service to MSs within a cell in a closed-loop MIMO system in a multi-cell environment, BSs allocate an optimal pre-codebook to MSs based on a channel state between the BSs and MSs.
The closed-loop MIMO system transmits data using information about a transmission channel of an MS serviced by a BS. In this case, since the BS cannot be aware of information about the transmission channel of the MS, the BS obtain the channel information, for example, channel state information (CSI), a channel quality indicator (CQI), and a precoding matrix index (PMI) from the MS.
In a multi-cell environment, the MS estimates a channel through which data is transmitted using a signal received from the BS. Next, the MS calculates the CQI using the estimated channel. The CQI is used to apply a modulation coding scheme (MCS) suitable for a channel state when the BS transmits data. Moreover, a channel coefficient which is most suitable for a channel state, that is, a precoding vector may be selected from a codebook used in an each cell using the estimated channel and a plurality of codebooks. Channel information obtained by the MS using the estimated channel is transmitted through a feedback channel between the BS and the MS. The BS selects the MCS and the precoding vector of the codebook using the channel information received from the MS. Thereafter, the BS transmits data to the MS using the selected MCS and the selected precoding vector of the codebook.
However, an MS in an edge of a cell receives a signal having weak strength from a BS of the cell to which the MS belongs and is subject to interference by an optimal pre-codebook allocated to an MS belonging to a neighboring cell by a BS of the neighboring cell. The MS in an edge of a cell decreases in performance by inter-cell interference caused by a pre-codebook.
Numerous studies on a closed-loop MIMO system have been conducted. A closed-loop MIMO scheme has been applied to a multi-cell environment as well as a single-cell environment. However, studies on a method for eliminating inter-cell interference by restricting a codebook used in each cell in order to reduce an influence caused by interference from other cells in the multi-cell environment is not active.
In a system using a closed-loop MIMO scheme in a multi-cell environment, BSs of each cell allocate an optimal codebook vector to MSs in consideration of a reception status of the MS within each cell. An MS in an edge of a cell is subject to interference by the optimal codebook vector allocated from a neighboring cell. The performance of the MS in an edge of a cell is decreased by the interference of the codebook vector.
Inter-cell interference mitigation proposed to eliminate interference caused by the codebook vector is applied to the case where the same codebook is used in each cell. Therefore, when a plurality of codebooks is used, inter-cell interference is not efficiently eliminated by the inter-cell interference mitigation.